1. Field of the Invention
This invention is concerned with micro-electro-mechanical systems (MEMS) and in particular with processes for fabricating MEMS devices.
2. Description of the Related Art
Micro-electro-mechanical systems (MEMS) fabricated from single crystal silicon by anisotropic etching are widely used for sensors (accelerometers, for example) and are of increasing commercial interest and importance for use in a variety of active devices, such as electrical switches, variable capacitors and inductors, and micromirrors for optical scanning and switching. A typical MEMS active device comprises a functional element that is anchored by a spring or hinge but suspended above a substrate so that it can be moved by an actuator, such as a capacitively driven comb structure. The moveable functional element might be a switch contact, capacitor plate or micromirror, for example. In addition to the small size desirable for portable equipment, MEMS devices offer the potential for faster response times, lower power consumption and reduced costs. Large cost benefits can be provided if the yield of functional devices per processed wafer is high.
An important potential application for MEMS is scanning mirrors, which are used in a wide variety of measurement and communications equipment including barcode readers, laser printers, confocal microscopes and fiber-optic networks. Compared to macro-scale scanning mirrors, MEMS micromirrors offer faster scanning speed, lower power consumption and reduced cost, and are enabling with respect to many new technologies. In particular, scanning micromirrors with high frequency optical switching capability are critical to development of advanced telecommunications systems.
The state of the prior art for MEMS wafer processing is illustrated by the fabrication process for a micromirror device with a comb actuator structure described in a recent publication (R. A. Conant, J. T. Lee, N. Y. Lau and R. S. Muller, p. 6, Proc. Solid-State and Actuator Workshop, Hilton Head Island, S.C., Jun. 4-8, 2000). For this device, one silicon layer comprises a coplanar circular mirror (550 μm diameter) and a moveable comb actuator suspended via a silicon torsion spring, which is connected to a stationary anchor. A second silicon device layer comprises a stationary comb structure whose teeth are immediately below the spaces between the teeth in the moveable comb. Capacitive charging of the teeth on the two combs by an applied voltage produces a force of attraction that tends to move the moveable comb and the attached micromirror, which are returned to their original positions by the torsion spring when the voltage is removed. The major steps in the prior art process for fabricating this mircromirror are as follows:                (1) A thermal oxide layer (0.2 μm thick) on a first silicon wafer is patterned via photoresist imaging and the exposed oxide and the underlying silicon layer are etched in a deep reactive ion etcher to form trenches (100 μm deep) that define the fixed comb teeth of the actuator. This silicon layer must be relatively thick since it serves as both the mechanical support and electrical connection for the finished device.        (2) The oxide layer on the comb teeth on the first silicon wafer is bonded to a thermal oxide layer (1.5 μm thick) on a second silicon wafer, which buries the fixed comb and mirror structure, and the bonded wafer pair is annealed (1100° C.) to increase the bond strength.        (3) The silicon layer above the oxide layer on the second wafer is then reduced to a thickness of 50 μm by grinding and polishing.        (4) The resulting silicon-on-insulator (SOI) wafer with the buried fixed comb structure is subjected to steam at 1100° C. to form an oxide layer (1.1 μm thick) on both outer silicon surfaces.        (5) Small windows are etched in the 50-μm silicon layer to expose two sacrificial buried comb structures, which are used to align masks for subsequent patterning.        (6) The oxide on the 50-μm silicon layer is patterned and etched to form the etch mask for the silicon mirror and moveable comb structure.        (7) The oxide on the thick silicon layer is patterned and the oxide and underlying thick silicon are etched to form a hole that serves as the optical path to the micromirror.        (8) The 50-μm silicon layer is etched to form the mircromirror and moveable comb actuator structure.        (9) The moveable mirror-comb structure is released via a timed wet chemical etch, typically hydrofluoric acid (HF) or buffered oxide etch (BOE), which removes the oxide layer between the moveable and stationary structures.        (10) An aluminum film is evaporated onto the micromirror to increase its reflectivity.        
This process for fabricating scanning micromirror devices, which is typical of the prior art for MEMS device fabrication, has several important disadvantages. In particular, the silicon-oxide-silicon bonding process requires temperatures in excess of 1000° C. and is very sensitive to particulates so that the wafers must be handled in at least a Class 10 cleanroom environment. Another significant disadvantage is that the through-wafer etching procedure used to open the optical path and provide access for removing the internal oxide layer greatly increases the processing time (3-5 hours), reduces the etching accuracy and device yield, and requires use of thicker photoresist with reduced feature resolution. The device yield is further reduced by the wet HF or BOE etching procedure required to remove the internal oxide layer and release the moveable parts of the device. This wet chemical etching procedure is difficult to control and is another major source of yield loss. In addition, the presence of deep etched features on both sides of the device complicates photolithographic processing and increases the likelihood of damage during handling. Also, use of the stationary comb silicon layer as one of the electrical connections limits options for device integration and in some cases may necessitate extra processing to electrically ground or connect floating elements. Furthermore, lack of precise alignment between the stationary and moveable comb structures for prior art processes further reduces the device yield and necessitates use of excessively high actuator voltages. Although typical of the prior art MEMS fabrication processes, all of these disadvantages may not apply equally to every device and every fabrication process.